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Crate TV60 Bad Output Transformer?

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  • Crate TV60 Bad Output Transformer?

    Received this Crate TV60 with no tubes and told that it may have a bad OT.

    With all OT wires disconnected, on primary I have continuity from center tap to outer legs, reading 70 ohms and 90 ohms. Across both outer legs I cannot get a reading. The meter readout keeps jumping. I thought I would see the sum of the 70 and 90 ohms.

    Secondary looks okay on resistance readings.

    I also noticed that F5 is blown. F6 is okay.

    I have attached schematic. I do not think this amp is worth putting money into but I wanted to see if I can bring it back to operation as I have nothing else to work on right now.

    Thoughts on issues? Thank you

    Click image for larger version

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  • #2
    I have a DMM that jumps when taking readings as you describe. In my case its noise being picked up by the windings. I also get it with some pickups with the same meter. Even if you get readings that add up you could still have a shorted turn and this is not detectable with a DMM. You either have to use the 'neon trick' or get hold of a ring tester that essentially does the same thing in a more fancy way. Transformer failures aren't at the top of my list when I'm looking for a fault, but when they are defective a shorted turn is quite common.

    The OT has flyback protection diodes so that's reassuring. It could have just had a shorted output tube on the side where the fuse had blown.

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    • #3
      Thanks so far. It looks like I am missing some of the schematic. I do not see any output tubes on the one I have.

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      • #4
        The reason for the measuring artefacts is the high primary inductance (full primary has 4 times the inductance of a primary half) and the counter EMF generated when connecting the meter. (I could explain in more detail if someone cares.) It is actually a good sign showing that there are most likely are no shorted turns and that the winding is not open.

        Just short the secondary while measuring primary DCR to facilitate a stable reading.
        Last edited by Helmholtz; 11-03-2019, 12:33 PM.
        - Own Opinions Only -

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        • #5
          Here is the TV-120 schematic.
          TV-120 Schematic.pdf.zip
          It may help.

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          • #6
            Interesting. Not sure I have seen a circuit where each output tube has its own fused protection. Help me understand what is the general concept here. Thank you

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            • #7
              The power tube cathodes are fused. It helps prevent a bad tube from damaging other components, like screen resistors, or in worst case, transformers.
              The Marshall JCM900 series had them mounted on the rear panel, with indicator lights showing that a power tube fuse had blown.
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


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              • #8
                Originally posted by Helmholtz View Post
                Just short the secondary while measuring primary DCR to facilitate a stable reading.
                That worked!

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                • #9
                  Just short the secondary while measuring primary DCR to facilitate a stable reading.
                  That is what I always did, facing this.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #10
                    Originally posted by Enzo View Post
                    That is what I always did, facing this.
                    Sure, the method is obvious to anybody who understands transformer (and meters).
                    Last edited by Helmholtz; 11-05-2019, 01:48 PM.
                    - Own Opinions Only -

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